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A risk assessment in support of a facility wind tunnel study.


Risks associated with the use of toxic chemicals can be evaluated, in part, by atmospheric dispersion modeling. Currently available mathematical models, however, are unable to accurately simulate complicated and dynamic flow patterns on and near buildings in a complex facility. The state-of-the-art approach for such a situation is wind tunnel modeling, a highly accurate process which measures exhaust chemical concentrations over a scale model in an enclosed tunnel with simulated wind conditions.

For this project, a 1:240 scale model of the facility was constructed to include buildings, terrain, and large trees within a 1,400-foot radius of the high hazard exhaust stacks of concern. Wind speeds and prevailing wind directions were recorded from an on-site meteorological tower and used to control conditions in the wind tunnel. Known concentrations of measurable chemicals released from the model's exhaust stacks provided information on atmospheric levels at various distances from the stacks, and video recording provided clear pictures of turbulence, travel, and dispersion of the plumes.

To understand the potential human health effects of various atmospheric levels of arsine, a toxic gas used in semiconductor processing, a risk assessment was conducted to develop acceptable exposure limits for the on-site and community populations which might be impacted by an accidental release of the chemical. These exposure limits were compared to downwind concentrations determined in the modeling. Both the risk assessment and the modeling it supported were important aspects of the facility's overall risk management program for hydride gases. It should be noted that there were no identified environmental concerns to be addressed for this particular chemical at this particular facility.

Risk Assessment and Toxicology

In 1983 the National Academy of Sciences (NAS) proposed what has become a classic and widely-applied model of risk assessment, the sequential components of which are hazard identification, dose-response assessment, exposure assessment, and risk characterization. Hazard identification determines whether a particular chemical is causally linked to certain health effects; dose-response assessment determines the relationship between magnitude of exposure and the probability of occurrence of the health effects; exposure assessment determines the extent of exposure with and without control mechanisms; and risk characterization describes the overall risks and uncertainties of the process. Accordingly, the health and safety professional acting as risk assessor, risk manager, and/or risk communicator must understand the toxicologic principles of hazard, exposure, and risk. These subjects are well covered in toxicology works such as references 1 and 2.

Hazard is an inherent property or ability of a substance to cause an adverse effect or event. Classes of hazards include physical hazards (e.g., compressed gases, explosives, flammables, organic peroxides, pyrophorics, unstable and water-reactive materials) and health hazards (e.g., asphyxiants, irritants, systemic toxins, reproductive toxins, carcinogens). The potential human health hazards associated with chemicals may range from mild and transient eye and skin irritation to more emotionally-charged effects such as decreased fertility and miscarriage.

Exposure defines contact of a hazard with a specific set of conditions. For physical hazards, exposure defines the conditions of contact (e.g., temperature) which will cause the hazard (e.g., flammability) to be expressed. For health hazards (toxicity), exposure defines contact of a biologic system (e.g., a person, an organ, a specific cell in an organ, a segment of genetic material) with a hazardous substance or condition. Among the exposure components that affect toxicity are route of exposure, frequency and duration of exposure, magnitude of exposure, genetic parameters (e.g., enzyme and hormone complements), dietary status, concurrent exposures, age, and ADME (absorption, distribution, metabolism, excretion).

The frequency and duration of exposure to toxic chemicals play an important role in toxicity. For many toxic chemicals, the adverse health effects associated with a single exposure are different from those associated with repeated exposures. Minimizing the duration of exposure and maximizing the time between exposures may reduce toxicity because of the body's defense mechanisms, including the ability to detoxify and/or excrete foreign substances. These defense mechanisms may, however, be overwhelmed when the frequency, magnitude, and/or duration of exposure is/are too great.

The magnitude of exposure (i.e., dose or concentration) also plays an important role in toxicity. Toxic responses are rarely all-or-none responses and can represent a wide range of effect. This range is often described by a dose-response relationship. The magnitude of exposure controls the dose-response relationship because it is a determinant of the amount of toxin at the body's target site. Magnitude of exposure, therefore, controls both the expression of a response (will it happen?) and the degree of response (how severe will it be?).

Risk is the probability that a hazard will be expressed, i.e., that an adverse effect or event will result from a given set of exposure conditions; it is a mathematical probability and so ranges from zero (no possibility of adverse effect or event) to one (certainty of adverse effect or event). It is important to keep in mind that risk is the mathematical product of hazard and exposure.

Multiplying by zero forces a product of zero, and the equation

RISK = HAZARD x EXPOSURE, therefore, means that an extremely hazardous substance may present little risk of adverse effect when it is handled under safe and prudent conditions, i.e., when the exposure component of risk is driven toward zero. Risk can similarly be driven toward zero by driving its hazard component toward zero by process design, i.e., by using the least amount of the least number of chemicals of the least possible toxicity.

Although risk assessment for communities is based on the same traditional components which form the basis for occupational and environmental risk assessment, major differences are often found in the populations at risk, in the development and application of exposure limits, and in the conditions under which exposure may occur. Many scenarios of community exposure are associated with transportation and handling incidents, i.e., situations in which traditional approaches to workplace health and safety may be ineffective. The hazard assessment must consider sensitive populations such as young children and the elderly, and the exposure assessment may require modeling of the chemical's release and dispersion characteristics.

The Covello-Merkofer risk assessment model includes the traditional components of risk assessment with important distinctions in order of conduct, in certain procedures, and in semantics. This model treats hazard identification as an independent and critical precursor to the risk assessment process, and proposes release assessment as the initial assessment process. Release assessment is relevant to community exposures because it addresses the potential of a risk source to release risk agents, quantifies the amount and probability of a release, and estimates the impacts of mitigating actions. In short, this model coordinates well with overall good facility management and planning. Detailed risk assessment methods are covered in reference 3, and additional information is provided in reference 4.

Exposure Scenarios

Exposure scenarios describe the incident conditions under which contact with a hazard may occur. In occupational settings, exposure scenarios describe potential contact of workers with the chemical and/or physical hazards in the workplace, usually on a task or process basis. Identifying exposure scenarios is a critical component of risk assessment for potential community exposure. The risk assessor must cautiously select scenarios which neither overstate nor understate the range and magnitude of possible incidents. For communities, the exposure scenarios are often developed from handling and transportation activities; most process-related community exposure scenarios should be addressed and controlled by responsible process safety management.

Populations at Risk

Occupational risk assessment generally treats workers as one population of basically uncompromised health status. Distinctions may be made when workers are known to be sensitive to identified hazards, i.e., when identified potential toxicity may overlap with relevant medical conditions or considerations. For example, women in the early stages of pregnancy may be considered a sensitive subpopulation in workers potentially exposed to radiation hazards.

Community populations potentially impacted by an industrial facility are typically substantially more varied than the resident occupational population. The risk assessor should identify the locations, population sizes, and proximities of schools, nurseries and daycare facilities, hospitals and other medical care facilities, recreation areas, public transportation facilities, and residences. A complete assessment of the potential impact of a facility incident should also include characterization of the nature, locations, and populations of other businesses. The location, proximity, and direction of evacuation and emergency response routes should be identified because receptors (subpopulations) which may not exist under normal circumstances may be formed as a result of the industrial incident under consideration.

Demographic data are important because they may indicate special sensitivities to specific hazards and because they are valuable for interpreting health outcomes and for initiating follow-up health investigations. For example, the functional liver enzymes which metabolize toxic chemicals are known to vary by such demographic parameters as ethnic group, sex, and age. Demographic information is available from the Bureau of the Census of the Department of Commerce, local governments, and health agencies. The reader is also referred to the Agency for Toxic Substances and Disease Registry (ATSDR), the agency of the U.S. Department of Health and Human Services, Public Health Service, charged under the Superfund Amendments and Reauthorization Act (SARA) to perform public health activities associated with actual or potential exposure to hazardous substances released into the environment. Petition by a person or persons may lead ATSDR to conduct a public health assessment for a facility or release incident (5). An ATSDR public health assessment differs from a risk assessment in that the public health assessment communicates health implications and further health actions while the risk assessment estimates quantitative risk and supports risk management decisions (e.g., control technology, response actions, cleanup levels).

Exposure Limits

Occupational exposure limits address the acceptable exposure levels or limits which are established and referenced to protect workplace populations from adverse health effects. Risk assessment for populations of workers generally considers one population of basically uncompromised health status. Exposure limits provide guidance for levels of contact with hazards that are considered toxicologically safe and/or aesthetically tolerable. It is important for health and safety professionals to research and understand the scientific basis for limits (e.g., target organ toxicity, eye irritation, odor complaint) as well as the numerical limits themselves. Limits may be based on structure-activity relationships, animal and in vitro toxicology testing, and/or epidemiologic evidence including occupational experience. For potentially toxic materials, the scientific basis of an exposure limit will be crucial to deciding whether to develop community limits from occupational limits. It should be noted that exposure limits for community populations can describe response actions (e.g., evacuation) as well as acceptable levels of exposure.

Dispersion Modeling

Having established the conditions under which a hazard may be released (i.e., the exposure scenarios) and the limit of that hazard to which community populations could be exposed (exposure limits), the risk assessor must estimate whether the hazard could, in fact, reach the community at level(s) of concern. Estimates can be based on dispersions models which simulate airborne concentrations of toxin and predict, e.g., plume width, direction of travel, and downwind distances and concentrations. Among the components which contribute to the modeling are the mass release and release rate, the duration of the release, phase at release, facility characteristics (e.g., building locations, sizes, shapes), surrounding terrain, and meteorological conditions. Mathematical dispersion modeling is bolstered by computer technologies and by wind tunnel studies in which releases from physical models can be simulated, measured, and visualized under varying conditions.

Risk Assessment in Support of a Facility Wind Tunnel Study

The toxic effects of arsine relevant to a handling or processing incident are red blood cell hemolysis (rupture) with subsequent anemia and renal failure; clinical signs include dizziness, weakness, nausea, jaundice, and decreased urine secretion. The mechanism of red blood cell hemolysis is increased cell fragility due to depletion of glutathione (GSH), a tripeptide found in animal and plant tissues. The toxicity of arsine can be overcome by sufficient de novo synthesis of GSH, a fact which helps to explain the time function of arsine toxicity and recovery (6). Although the mechanism of arsine toxicity is important to understanding the relevance of occupational exposure data to community populations, a comprehensive review of arsine and arsenic toxicity is beyond the scope of this article, and the reader is referred to references 6 and 7 for more complete discussions.

For a facility which handles a toxic gas such as arsine, two reasonable scenarios are: 1) release from a facility roof vent, under ventilation controls, due to cylinder or equipment failure; and 2) release from a compressed gas cylinder, without ventilation controls, due to a delivery accident. Further scenario detail must be developed on a site- or incident-specific basis. For example, for a process which has a maximum allowable quantity of arsine of 1,800 g, reasonable release scenarios could range from less than one minute (cylinder contents released without limiting orifice) to 140 minutes (13 grams/minute released through 0.010-inch limiting orifice). The duration of release, i.e., the duration of potential exposure, is relevant to health-based exposure limits for potentially affected populations.

For this project, populations of concern included on-site workers, particularly those not directly handling arsine, and off-site (community) populations. The following discussions provide basic considerations in risk assessment for these groups by comparison with occupational equivalents. The discussions are intended to be representative rather than comprehensive and include information presented in references 7 and 8. Acute and chronic exposure scenarios are considered for each population.

Available Community Exposure Limits for Arsine

The Santa Clara County California Fire Chiefs Association has established a procedure for calculation of a Community Exposure Limit (CEL) which is the "minimum toxic gas concentration level for a single gas at which protection or evacuation of the community is recommended" (7,8). The CEL is the lowest of the short-term exposure limit (STEL) of the American Conference of Governmental Industrial Hygienists (ACGIH), 2X the ACGIH threshold limit value (TLV), or 1% of the [LC.sub.50] (the lowest concentration lethal to 50% of a group of experimental animals). For arsine, the CEL is 200 ppb based on the [LC.sub.50].

The National Academy of Science Emergency Exposure Guidance Level (EEGL) for arsine is a 1-hour exposure limit of 1,000 ppb and the National Institute for Occupational Safety and Health (NIOSH) Immediately Dangerous to Life and Health level (IDLH) is 6,000 ppb. The IDLH references "conditions that pose an immediate threat of severe exposure to contaminants...which are likely to have adverse, cumulative, or delayed effects on health" (7,8). The use of an IDLH as the basis for a community exposure limit is tenuous because NIOSH intended the limit to apply to workplace respirator selections and not to 30-minute exposure standards. Although not yet applied to arsine, another methodology of community exposure limit development is provided by the American Industrial Hygiene Association's Emergency Response Planning Guidelines (ERPGs). For each chemical of interest, three 1-hour ERPG levels are calculated: one level to protect against mild, transient effects or odor, a second level to protect against reversible effects which do not impair escape ability, and a third level to protect against effects which are not life-threatening.

Calculated Exposure Limits for Arsine

The following discussion on arsine exposure limits is based on the exposure scenarios described above (release under ventilation controls and release without ventilation controls) and the estimated time range of less than one minute to about 140 minutes. For this discussion, acute exposure is limited to hours and chronic exposure is limited to days, weeks, or months. For comparison with the limits described below, note that arsine is generally considered to be instantly lethal to humans at 250,000 ppb, lethal in 30 minutes at 25,000-50,000 ppb, and lethal at 10,000 ppb for longer durations. Additionally, EPA has established 600 ppb as the level of arsine for which a 30-minute exposure is unlikely to result in adverse health effects in humans (1/10th of the National Institute for Occupational Safety and Health Immediately Dangerous to Life and Health level) (3,4,6,8,9,10).

Calculations for Acute Exposure - On-Site Populations

Acute exposure of employees to arsine could be associated with failure of equipment, e.g., a cylinder, regulator, valves, piping. Exposure limits applicable to acute exposure can be based on the excursion limit approach of the American Conference of Governmental Industrial Hygienists (ACGIH) and on the equality of the OSHA-regulated permissible exposure limit (PEL) and the ACGIH threshold limit value (TLV). Further justification for this approachach is provided by the similarity of the TLV for arsine to the TLV for other inorganic arsenic compounds, i.e., protection against acute and cumulative hematological and renal effects and against oxidation of arsine to other arsenic compounds.

For compounds for which ACGIH finds the toxicology data insufficient to calculate a short-term exposure limit (STEL), excursion limits from the TLV are based on practical application of a log-normal concentration distribution. The excursion limit for arsine is 150 ppb (3X the TLV of 50 ppb); this 150 ppb limit may not be exceeded for more than 30 minutes during a workday, and so is considered generally applicable to acute exposure of employee populations.

Calculations for Acute Exposure - Community Populations

An acute exposure limit for community populations could be based on the excursion limit approach described above with application of a 3X uncertainty factor to account for greater sensitivity of the community population relative to the employee population. The resultant exposure limit is 50 ppb. The 3X uncertainty factor is considered sufficient due to the mechanism of action of arsine, i.e., arsine toxicity would be expected to be without great variation in human populations because of the mechanism of red blood cell effects.

An exposure limit to protect community populations against the hematological and renal effects of arsine can also be based on animal toxicology studies and National Academy of Sciences methodologies for noncarcinogenic effect; the No Observed Adverse Effect Level (NOAEL) approach is described in reference 9. For arsine, a NOAEL was identified in mice exposed to 500 ppb for one hour; this was the lowest concentration tested (10). Uncertainty factors of 3X to account for interspecies extrapolation and 10X to account for varying sensitivity within the community population are applied to the NOAEL. The resultant calculated exposure limit is 170 ppb.

Calculations for Chronic Exposure - On-Site Populations

Chronic exposure to arsine is not expected for the employee population that handles arsine at a well-managed facility. This conclusion is based on the safety processes and systems which should be in place, including procurement review, process safety review, engineering control equipment such as gas cabinets, and toxic gas monitoring and calibration programs. Chronic exposure to arsine for the remaining employee population at the facility is equivalent to chronic exposure for the community populations.

Calculations for Chronic Exposure - Community Populations

Chronic exposure could be associated with a process-related release of arsine over days, weeks, or months. A NOAEL was identified in rats and mice exposed to arsine for three months at 6 hours/day, 5 days/week (10). The calculated exposure limit from this study would be 0.8 ppb based on an NOAEL of 25 ppb and uncertainty factors of 3X to account for interspecies extrapolation and 10X to account for varying sensitivity within the community population. Adjustment from experimental exposure to continuous exposure is not made because the experimental exposure pattern more closely approximates the pattern of arsine use during normal business hours.

This toxicology study was also used by EPA to calculate a reference concentration (RfC) for chronic (lifetime) exposure to arsine. An RfC of 0.015 ppb was calculated from the NOAEL of 25 ppb (adjusted from 25 ppb under discontinuous experimental exposure to 8 ppb under postulated continuous exposure) and uncertainty factors of 3X for interspecies extrapolation, 10X for intraspecies sensitivity, and 10X for database deficiencies.

The 1-hour animal NOAEL-derived limit of 170 ppb discussed above could also be used as the basis of a chronic exposure limit. Application of a 10X uncertainty factor to account for application of acute data to chronic exposure would result in a limit of 17 ppb.

Arsenic and certain arsenic compounds are classified as known or suspected human carcinogens by several recognized and accepted scientific and regulatory organizations. OSHA regulates arsenic and inorganic arsenic compounds as occupational carcinogens with a PEL of 0.010 mg/[m.sup.3] (as arsenic) applicable to a working lifetime. This PEL is equivalent to 3 ppb arsine. Application of a 10X uncertainty factor to account for varying sensitivity within the community population would result in an exposure limit of 0.3 ppb arsine. It is stressed that this calculation is conservative in that the OSHA PEL is based on exposure over a working lifetime and that insufficient data are available on which to base an evaluation of the potential carcinogenicity of arsenic from exposure to arsine for hours, days, weeks, or months.


Table 1 summarizes the exposure limits described in this article. This overview presents arsine exposure limits which 1) range from 0.015 ppb to 170 ppb, 2) are based on various toxicological endpoints, 3) apply to different exposure durations, and 4) may leave the risk assessor confused as to which is the right exposure limit. This decision point is the dividing line between risk assessment and risk management. One option is to select the most conservative (lowest) exposure limit, an option which may be appropriate for certain communities and/or political situations. Another option is to use the limit of detection (in the range of 5-50 ppb arsine based on paper tape methodology), an approach successfully utilized in many historical regulatory and risk management activities. The limit(s) of control technologies such as scrubbers can also be utilized.

In this project, wind tunnel modeling confirmed that off-site (community) populations would not be impacted by an accidental release of the hydride gas arsine under the scenarios considered. For on-site employees, potential exposures were similarly determined to be below the most stringent detection limit of 5 ppb. Separate assessment and control measures were addressed for those employees directly involved in the handling and use of arsine cylinders.

Although it may not be comforting to realize that there is not necessarily one right answer, risk assessment for community exposure must still be based on sound understanding of the underlying toxicology of hazard, exposure, and risk. Careful attention must be paid to the unique considerations [TABULAR DATA FOR TABLE 1 OMITTED] of the populations at risk, reasonable exposure scenarios, applicable exposure limits, and dispersion modeling. Risk management decisions and risk communication activities must be planned and implemented by well-informed professionals who understand the unique challenges of risk assessment for community exposure.

In occupational settings, exposure, and therefore risk, may be controlled or minimized with prudent work practices, engineering controls (e.g., laboratory design, exhaust ventilation), and, if necessary, personal protective equipment. The principles of prudent work practices and the processes of hazard minimization and equipment design also apply to control of potential community exposure from transportation and handling incidents, but workplace engineering controls and personal protective equipment are likely to be ineffective protection beyond the workplace. Also, factors that affect toxicity must be included in process design efforts because the hazard of concern may vary with the conditions under consideration, i.e., routine vs. unexpected (catastrophic) events. Potential community exposure scenarios must be developed, and plans must be formulated for control, mitigation, emergency response, and remediation. Risk management and risk communication are integral to these activities.

The final task of risk assessment for community exposure is risk communication. When compared to an employee group, the community group may be less formally trained with regard to chemical hazards, may feel less well-protected than an employee provided with protective equipment, and may be more outraged by possible involuntary risks. In non-occupational settings, acceptable risk is often a matter of judgment and/or a balance between risk and benefit. For example, high risk may be acceptable in life-saving drugs but unacceptable in food additives. Also, voluntary risks such as swimming may be judged more acceptable than involuntary risks such as exposure to airborne pollutants or facility releases. Risk communication is often the most challenging component of a comprehensive risk program. Additional information is available in references 11, 12, 13, and 14.


1. Amdur, M.O., J. Doull, and C.D. Klaassen (eds.) (1991), Casarett and Doull's Toxicology. The Basic Science of Poisons, 4th Edition, Pergamon Press, New York.

2. Sullivan, J.B., Jr. and G.R. Krieger (eds.) (1992), Hazardous Materials Toxicology. Clinical Principles of Environmental Health, Williams & Wilkins, Baltimore, Md.

3. Covello, V.T., and M.W., Merkhofer (1993), Risk Assessment Methods. Approaches for Assessing Health and Environmental Risks, Plenum Press, New York.

4. Center for Risk Analysis (March 1994), A Historical Perspective on Risk Assessment in the Federal Government, Harvard School of Public Health, Boston, Mass.

5.55 Federal Register 5136, February 13, 1990.

6. American Conference of Governmental Industrial Hygienists (1993), Documentation of the Threshold Limit Values and Biological Exposure Indices, 6th Edition, ACGIH, Cincinnati, Ohio.

7. U.S. Dept. of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry (1992), Public Health Assessment Guidance Manual, Lewis Publishers, Chelsea, Mich.

8. Lowe, J., G.R. Krieger, S.R. Radis, and J.B. Sullivan, Jr. (1992), "Assessing Community Risk from the Sudden Release of a Toxic Gas," In: Sullivan, J.B., Jr. and G.R. Krieger (eds.), Hazardous Materials Toxicology. Clinical Principles of Environmental Health, Williams & Wilkins, Baltimore, Md. pp. 451-462.

9. National Academy of Sciences (1983), Risk Assessment in the Federal Government. Managing the Process, National Academy Press, Washington, D.C.

10. U.S. Environmental Protection Agency, Integrated Risk Information System (IRIS), Inhalation RfC Assessment (RDI) for Arsine, IRIS Number 655, March 1, 1994, National Library of Medicine (on-line), Bethesda, Md.

11. Sandman, P.M. (1991), Risk = Hazard + Outrage, American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio.

12. Journal of Occupational and Environmental Medicine (formerly Journal of Occupational Medicine), Williams & Wilkins, Baltimore, Md.

13. Environmental Health Perspectives, U.S. Public Health Service, U.S. Dept. of Health and Human Services.

14. Risk Analysis, An International Journal, Plenum Publishing Corp., New York.

Lisa Brooks, Ph.D., EH&S Manager, AT&T Bell Laboratories, 600 Mountain Ave., P.O. Box 636, Murray Hill, NJ 07974-0636.
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Author:Brooks, Lisa
Publication:Journal of Environmental Health
Date:Jun 1, 1995
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